This proposal examines the effect of aerobic fitness training on human cognition, brain structure, and brain function of older adults. Our study is prompted by the encouraging effects of fitness training on human cognition, (Coxcomb &Kramer, 2003) and more recently human brain structure and function (Colcombe et al., 2003, in press). Such effects are of great interest both for what they tell us about the nature of cognitive and brain aging and also for their public health implications. The main specific aims of our studies include: 1) To test the hypothesis that improvements in aerobic fitness of older adults will lead to improved performance on a variety of cognitive processes, and more specifically those processes that are supported, in large part, by frontal regions of the brain. 2) To test the hypothesis that the improvements in cognitive processes engendered by enhanced aerobic fitness will be supported by changes in underlying neural circuits, as inferred from patterns of fMRI activation obtained in a 3 tesla scanner. We hypothesize that the changes in the patterns of fMRI activation that result from improvements in aerobic fitness will approach those activation patterns exhibited by young adults. 3) To test the hypothesis that improvements in aerobic fitness, over the course of a 1 year intervention, will result in increases in gray and white matter volume in regions of the human brain (i.e. primarily frontal, medial temporal and parietal) that have previously been demonstrated to show sparing in cross-sectional studies. 4) To test the hypothesis that increased levels of insulin-like growth factor I (IGF1) and decreases in markers of inflammation (i.e. C-reactive protein &interleukin-6) engendered by improvements in aerobic fitness will moderate the fitness benefits observed on measures of cognition, brain function and structure.